1. Field of the Invention
The invention relates generally to integrated circuit packages, and more specifically to an integrated circuit package having a top cavity die attach area while sinking heat through the bottom of the package.
2. Description of Related Art
Packages for integrated circuits have evolved over the years from the simplistic single and dual inline packages (SIP) and (DIP) respectively, through the more sophisticated and denser pin grid array (PGA), staggered pin grid array (SPGA), and ball grid array (BGA). The so-called tape carrier package (TCP) is used in small form-factor products such as laptop computers, due to its small footprint and low clearance characteristics. The TCP is produced through the so-called Tape Automated Bonding (TAB) process that is pervasive in portable electronics. An exemplary, but not exclusive application of the TCP can be found in the mobile Pentium.RTM. processor from the Intel Corporation of Santa Clara, Calif.
Referring now to FIG. 1, a TCP device 10 (such as the mobile Pentium.RTM. processor) is constructed from a flexible tape 12 that is laminated with a copper foil on its bottom. The flexible tape 12 has an opening 14 formed near its center for attachment of a die 15. The copper foil is photo-imaged and etched forming an arrangement of pads around the opening 14 with gold plated electrical traces extending therefrom to conductive fingers 16 formed at the outer borders of the tape 12. The fingers 16 are bent in a gull-winged fashion for attachment to a circuit board 18. Special "gold bumps" 25 are deposited on standard aluminum bond pads formed around the top periphery of the die 15. The gold bumps 25 are then attached to the arrangement of pads formed on the bottom of tape 12 through so-called thermal compression bonding. After the die 15 is attached, a blob of encapsulant such as polyimide siloxane 20 is applied to the opening 14 to protect and seal the die 15.
To attach the TCP 10, special tooling presses the gull-winged shaped fingers 16 to the circuit board 18 and applies heat to fuse the fingers 16 to pads formed on the circuit board 18. In its installed position, the bottom of the die 15 is directly attached with a thermally conductive adhesive 26 to the top of a plurality of thermal vias 22 (i.e. metal-plated through holes) formed through circuit board 18. The bottom of the thermal vias 22 are coupled to the top of the heat sink 24 through thermal plane 27 and thermal grease. The TCP device 10 sinks heat through its bottom due to its die 15 being in direct contact with thermal vias 22 on the circuit board 18, and ultimately through the heat sink 24 disposed underneath the circuit board 18. Alternatively, a heat pipe (not shown) could be coupled between the thermal plane 27 and a remotely located heat sink 24.
By way of further background, a standard Ball Grid Array (BGA) package (not shown) is a piece of "circuit board-like" planar laminate material (however much thicker and sturdier than the TCP tape) with its bottom (i.e. the side towards the main circuit board in its mounted position) having a die attach cavity formed thereon with electrical traces etched from the die attach area to an array of solder balls (ergo BGA). After the die is affixed to the die attach area, the aluminum bond pads are bonded out to the etched traces on the laminate material by well known wire bond techniques and a resin encapsulant is applied over the die for protection. A thin piece of metal acting as a heat spreader is attached to the top of the BGA package and a heat sink is typically attached to the heat spreader to further dissipate heat.
The BGA device is attached to the main circuit board by placing it so that its solder balls are aligned with attachment pads formed on the main circuit board and passing the entire circuit board through an oven that re-flows the solder balls. The BGA package, which has a small footprint, low clearance, and is easier to install than the TCP, sinks heat through the top mounted heat spreader in contact with a top mounted heat sink--as is conventional with most integrated circuit packages. Consequently, a drawback with switching a design with a part having a TCP to a BGA package in a tight form-factor product (e.g. a laptop computer), is that it requires extensive mechanical and thermal changes to the internals of the product (e.g. case size, circuit board mounts, and heat sink/heat pipe designs).
Another drawback with the TCP is that it requires special tooling to: cut the assembly from the tape, form the leads into a gull wing like shape, and to place and solder the assembly onto the circuit board. Moreover, the thin flexible tape used as the substrate in a TCP is subject to warp under heat. Consequently, an attempt to adapt a TCP into a BGA arrangement is undesirable since standard re-flow soldering techniques would most likely warp the tape substrate thus skewing the solder balls off of the attachment pads--making attachment unreliable. Moreover, the attachment process becomes more complex and ostensibly more expensive if stiffeners are added to the tape substrate after the part is already formed and cut with the special TCP tooling.
Another drawback with TCP is the die is required to have special gold bumps deposited on its aluminum pads for thermal compression bonding--adding complexity and cost to the product.
Yet another drawback with the TCP is that the yield rates for successful circuit board installation are typically much lower as compared to more conventional packages such as the BGA since, among other things, the die which is brittle, is directly attached to the main circuit board which may be subject to twisting and shock.
Yet another drawback, particularly with devices which have high power consumption requirements such as a processor, is that heat is dissipated through thermal vias which are formed in the circuit board--potentially weakening or causing damage to surrounding components.
Accordingly, it can be seen from the foregoing, that there is a need for an alternative integrated circuit package that minimizes mechanical and thermal changes to systems originally designed for low profile TCP devices.